1. Field of the Invention
This invention relates to temperature setpoint circuits, and particularly to temperature setpoint circuits with hysteresis.
2. Description of the Related Art
Many circuits have been devised to sense ambient temperature and to toggle an output signal when a predetermined setpoint temperature is reached. Such circuits often employ resistors or bipolar transistors having characteristics which vary in a predictable way with temperature. However, such circuits suffer from a number of shortcomings. For example, the temperature setpoint may vary due to a variety of manufacturing variables, such as sheet resistance variations, current density errors, and variations in the transistors' base-to-emitter voltages from lot to lot.
One bipolar transistor-based temperature setpoint circuit is described in U.S. Pat. No. 5,821,741 to Brokaw, an embodiment of which is shown in FIG. 1. A transistor Q1 and a current source 10 providing a current I1 are sized such that Q1 operates at a predetermined current density, and a transistor Q2 and a current source 12 providing a current I2 are sized such that Q2 operates at a different, smaller current density. The circuit is arranged such that I1 and I2 are proportional-to-absolute-temperature (PTAT) currents. A transistor Q3 is arranged to force Q1 to conduct I1. A resistance R1 is connected between the bases of Q1 and Q2, such that the difference in their base-emitter voltages (ΔVbe) appears across R1. A resistance R2, and a setpoint resistance Rset, which may be external to the IC, form a voltage divider with R1. Rset is selected to produce a desired temperature setpoint.
The circuit is arranged such that Q2 is turned on and conducts a current equal to I2 when:    ΔVbe=(kT/q)ln(J1/J2), where J1 and J2 are the current densities in Q1 and Q2, respectively.    The temperature T at which ΔVbe=(kT/q)ln(J1/J2) is the said circuit's setpoint temperature Tsp.
A transistor Q4 is connected such that it is turned on when Q2 is turned off, and vice versa; the collector of Q4 provides the setpoint circuit's output voltage Vout. When the temperature is less than Tsp, Q2 is off, Q4 is on and Vout is low, and when the temperature is greater than Tsp, Q2 is turned on, Q4 is turned off, and Vout goes high.
In some temperature setpoint applications, it is desirable that the circuit exhibit some hysteresis around the setpoint temperature, to prevent multiple transitions of Vout when the temperature crosses the setpoint. In U.S. Pat. No. 5,821,741, this is accomplished by adding small value resistors in series with the emitters of Q1 and Q2. Since I1 and I2 are PTAT currents, the voltages developed across the series resistors at the set point are also PTAT. These voltages serve to offset the base-emitter voltages of Q1 and Q2, thereby causing the circuit's trip point to be shifted. When Q2 is off, a controlled PTAT current is switched into the resistor in series with Q2's emitter, which further shifts the trip point. However, when Q2 is on, the controlled PTAT current is disabled and the trip point shifts back, thereby creating some hysteresis around Tsp.
As a practical matter, these small value resistors connected in series with Q1 and Q2 are difficult to fabricate, and to make match the other circuit components. They are also difficult to trim, particularly since it is difficult to separate their effect from the set point value.